...firm-specific idiosyncratic volatility does not average out because of the presence of very large firms. We refer to this as the ‘granularity’ hypothesis. The second – from Acemoglu et al. (2012) – is that idiosyncratic shocks contribute to aggregate fluctuations because input-output linkages generate comovement between firms. We refer to this as the “linkages” hypothesis.Some spiffy graphs in their paper show;
... direct corroboration in the data for the mechanisms behind both the ‘granularity’ and the ‘linkages’ hypotheses. Sectors that are populated by firms that are more interconnected with the rest of the economy; and more concentrated contribute a disproportionate share of aggregate volatility relative to what we would expect in a ‘symmetric’ economy.
Their coup de grâce;
Trade integration has the potential to make the largest firms even larger (di Giovanni and Levchenko, 2012). Likewise, consolidation across industries -- for instance via mergers and acquisitions -- also leads to a fatter tail in the firm-size distribution. These two structural changes amplify granular fluctuations, making business cycles more sensitive to individual firms’ shocks. At the same time, the boundaries of the firm are changing and production processes are becoming more fragmented. Some activities that used to be internal to the firm are now outsourced. This fragmentation takes place both within and across borders, and within and across sectors, adding further scope for shocks to individual firms to propagate throughout the economy as well as across countries.